1. Field of the Invention
The present invention relates to a thermally-developable light-sensitive element, and more particularly to a high sensitivity thermally-developable photographic element in which the storage stability of the light-sensitive element is improved such that the photographic characteristics possessed by the light sensitive element immediately after production are retained (hereinafter "fresh characteristics"), even under high humidity and/or high temperature conditions.
2. Description of the Prior Art
Photography using silver halide has been carried out most widely hitherto, because silver halide photographic elements have superior photographic properties such as sensitivity and graduation to those possessed by light sensitive elements used in electrophotography or diazo photography. Recently, much research and development on methods for obtaining an image without using a wet processing with a developing solution or the like by changing the processing to a dry processing by heating or the like in the photographic process of forming an image on light-sensitive silver halide photographic elements has been carried out.
Of these light-sensitive photographic elements on which a photographic image can be formed using such a dry processing system, a thermally developable light-sensitive element using a composition containing an essential components, a silver salt of an organic acid, a small amount of silver halide and a reducing agent, e.g., as disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075, has been considered as being at present the most advanced photosensitive element. This thermally developable light-sensitive element is stable at normal temperature to about 50.degree. C. However, silver is produced in the photographic element, when heated usually to about 80.degree. C. or higher, more preferably to 100.degree. C. or higher, following image-wise exposure to light, due to an oxidation-reduction reaction between the silver salt of the organic acid as an oxidant and the reducing agent in the light-sensitive layer with this oxidation-reduction reaction being caused by the catalytic action of metallic nuclei formed by the exposed silver halide in proximity to the oxidant and the reducing agent therein. As a result, the exposed areas of the thermally developable light-sensitive layer are rapidly blackened by the production of silver which results in an image formed due to a difference in the contrast between the exposed areas and the unexposed areas (background) thereof.
In the light-sensitive system, the silver halide remaining in the light sensitive element following development is not stabilized against light but allowed to be discolored by light. In spite of the discoloration, the system provides the same effects as those obtained in a system where silver halide is stabilized against light. The reason is because the silver salt present in the light sensitive element comprises a minor amount of silver halide and a major amount of a white or slightly colored organic silver salt which is relatively stable to light and is not thereby discolored, and even if a minor amount of silver halide is discolored by light, the light-sensitive layer remains white or only slightly colored overall, so that the minor amount of discoloration scarcely adversely affects the visual appearance.
The above-described thermally-developable light-sensitive element usually comprises a support having coated thereon the above-described thermally developable light-sensitive layer containing a silver salt of an organic acid, a silver halide and a reducing agent. A variety of materials can be employed as the photographic support and used appropriately depending upon how the thermally developable light-sensitive element is observed.
For example, various kinds of synthetic resin film supports which are disclosed in U.S. Pat. No. 4,039,334 all are employed as a support for a transparent type thermally-developable light-sensitive element. In contrast to this, where a reflection type thermally-developable light-sensitive element ordinarily used for copying documents is to be produced, a paper is most conventionally used as a support thereof due to low cost and ease of handling or the like.
In order to increase the sensitivity of the above-described thermally-developable light-sensitive element, a method which comprises employing a silver halide having a large grain size in the same manner as the sensitization of a conventional gelatin silver halide emulsion which is wet-processed is most effective.
Unexpectedly it was discovered that the employment of a silver halide having a large grain size, particularly an average grain size of about 0.05.mu. or greater in a thermally-developable light-sensitive element results in a deterioration of the storage stability of the fresh photosensitive element under conditions of high humidity and results in a reduction in the maximum image density in particular (see Comparative Example 1 give hereinafter).
As set forth hereinbefore the term "the storage stability of the fresh light sensitive element" as used herein means the ability of the photographic characteristics exhibited by a thermally-developable light-sensitive element immediately after the production thereof to be retained after storage for a long period of time. In addition, it has been found that the storage stability of the fresh light sensitive element tends to deteriorate more markedly where the support used in a gas permeable material such as a paper.
The above-described defects have been minimized by employing a support having thereon a subbing layer comprising a specific copolymer according to the present invention.
It was indeed known hitherto that a thermally-developable light-sensitive element can also contain various photograhic layers other than the thermally-developable light-sensitive layer, such as an uppermost protective layer, a subbing layer or a backing layer coated on the opposite surface of the support to the light-sensitive layer. However, it is quite unknown how the subbing layer of these photographic layers affects the thermally-developable light-sensitive element. In addition, even though U.S. Pat. No. 4,039,334, 3,761,279, etc., disclose use of various kinds of natural or synthetic polymers as a polymer for the subbing layer, all of these U.S. Patents are completely silent about what type of polymers are suitable for use in thermally development type photography and what effects are obtained thereby. These polymers disclosed therein also include those which have a weak heat resistance and therefore, may be transformed on heating, such as polyethylene and the like as disclosed in U.S. Pat. No. 3,761,279.
Further, generally the subbing layer is often used for the purpose of increasing the adhesion between a support and a specific layer to be adhered thereto (e.g., a photographic emulsion layer in a conventional silver halide photo-sensitive element and a thermally-developable light-sensitive layer in a thermally-developable light-sensitive element). However, the subbing layer in a thermally-developable light-sensitive element is not employed in many cases, since the thermally-developable light-sensitive layer by itself can adhere strongly to a support.
Still further, Japanese Patent Application (OPI) No. 43130/1976 discloses a thermally-developable light-sensitive element which comprises a support whose surface carries thereon a thermally-developable light-sensitive layer and in addition, the back thereof has a polymer layer thereon. This light sensitive element is prepared for the purpose of improvement in the storage stability of the fresh light sensitive element, when the light sensitive element is stored such that the above-described polymer backing layer and the thermally-developable light-sensitive layer are piled on each other and are in contact with each other (e.g., when the light-sensitive element is rolled up). However, it was surprising that the application of a polymer layer to the back of the support did not resolve the above-described defects (see Comparative Example 2 given hereinafter).